Dielectric materials are used in manufacturing integrated circuits. As described in VLSI TECHNOLOGY, second edition, by S. M. Sze, 1988, on page 234, dielectric materials have many purposes, such as insulation between conducting layers, diffusion and ion implantation masks, diffusion from doped oxides, capping doped films to prevent the loss of dopant, gettering impurities, and as passivation to protect devices from impurities, moisture and scratches.
Phosphorus doped silicon dioxide (PSG) is one such dielectric material. It is frequently used as an insulator between polysilicon gates and top metallization. U.S. Pat. No. 5,112,762, issued May 12, 1992, illustrates in FIG. 3 a high density dynamic random access memory, DRAM, device having a multilevel oxide (MLO) oxide layer 30 that could be formed of PSG.
Phosphorus in an interlevel oxide isolation between conductive layers act as an ion migration barrier, but the main reason for doping is to decrease the reflow temperature as explained in SEMICONDUCTOR INTEGRATED CIRCUIT PROCESSING TECHNOLOGY by Runyan and Bean, 1990, pages 143-145. Reflowing the PSG helps reduce the topography which is good for depositing metal. A problem in interlevel connections is that etching process for forming contact apparatuses through the interlevel oxide may produce openings with sharp edges. If the softening point of the interlevel oxide can be reduced enough, then it can be reflowed after the openings are made in order to round off the abrupt edges. Similarly, after an oxide is deposited over a series of leads, the abrupt shoulders can be smoothed by reflowing. PSG softens and reflows at a relatively high temperature between 950 to 1100 C. Typical dopant concentrations in PSG are about 6 to 8 wt. % phosphorus concentration. See VLSI TECHNOLOGY page 234 and pages 255-258.
Another dielectric material commonly used is borophosphosilicate glass (BPSG). Multilevel oxide (MLO) layer 30 of the above U.S. Pat. No. 5,112,762 could be formed of BPSG. BPSG is formed by adding boron to PSG. Adding boron to PSG provides an advantage of lowering the reflow temperature of PSG. Typical reflow temperatures for BPSG are between between 800 to 900 C. Typical dopant concentrations are 1 to 4 wt. % boron and 4 to 6 wt. % phosphrous. See VLSI TECHNOLOGY page 257.
It is desirable to lower the reflow temperature of dielectric films to avoid disturbing modern shallow junction transistor devices. Modern VLSI circuit devices, such as the 16M bit DRAM in the above U.S. Pat. No. 5,112,762 have shallow junction transistors in the memory array and periphery. The source/drain regions of the transistors are formed, as for example, by ion implantation before applying a dielectric material such as BPSG. Thus, the reflow is commonly referred to as a "back end" operation; that is, an operation occurring after formation of the transistors. Exposing the source/drain regions to too high a reflow temperature in the back end adversely disturbs the dopant concentration in the source/drain regions.
One way to achieve lower reflow temperatures is to increase the content of the dopant species. By increasing the boron and phosphorus concentration in a BPSG layer, the reflow can be lowered. However, this is undesirable because too high levels of these dopants produce a deleterious influence on the film properties. Phosphorus concentrations above 7%-8% wt. may cause corrosion of the aluminum metallization by the acid products formed from the reaction between the phosphorus in the oxide and atmospheric moisture. Boron contents above about 4% make a glass unstable in high humidity. Bubbles of volatile phosphorus oxides and crystallites of boron-rich phases can occur unless the dopant concentrations are carefully controlled.
It is desirable therefore, and accordingly an object of the invention, to provide a dielectric material having low temperature reflow characteristics.
It is an additional object of the invention to lower the reflow temperature of doped dielectric films.
It is a further object of the invention to provide an improved BPSG film.
Other objects and advantages of the invention will be apparent to those of ordinary skill in the art having reference to the following specification and drawings.